1. Field of the Invention
The invention relates to valves and more particularly to improved methods for fabrication of valves and valve seats.
2. Description of the Prior Art
Valves are widely used to control the flow of liquids. Typical prior art valves have diameters ranging from fractions of inches to two to three feet. Valves in the various size categories present unique assembly and repair problems. For example, relatively small valves may be easily removed from service and replaced rather than repaired. By contrast, large valves, such as those typically used in nuclear reactor feed water and steam systems are typically constructed using exotic materials such as stainless steel and are welded to associated equipment, such as pipes. Hardened valve seats are necessary in such uses. Under such circumstances repairing a faulty valve seat is a time consuming, expensive and sometimes hazardous feat. For example, typical techniques for applying hard surfacing material to surfaces as part of either a production or a repair process requires complicated equipment which is difficult to use outside the factory environment. These characteristics of prior art technology make it necessary to remove the valve from service in order to effectuate repairs or utilize an expensive and complicated repair technique.
Some materials which have characteristics which are desirable for valve seats also have characteristics making preformed valve seats made of these materials difficult to install or remove from valves. For example, preformed ceramic valve seats are not readily attachable to the body of a valve by fusion processes, such as conventional welding. Attempts to weld a preformed ceramic valve seat in place often destroys the valve seat by cracking. A threaded preformed ceramic valve seat is brittle and often breaks. Additionally, processes used to form layers of ceramic materials on the body of valves using fusion processes (such as welding) which can be machined with the valve attached to associated equipment to produce suitable valve seats are complicated and expensive. These characteristics of prior art techniques for producing valves having hardened valve seats and for repairing such valves are essentially incompatible with the need for a reliable and low cost valve seat which is easy to replace for purposes of repairing the valve. Other hard materials useful for valve seats have similar problems.
The invention which is the subject matter of this invention substantially reduces the above discussed problems with prior art valves utilizing hardened valve seats and repair processes for such valves. More specifically, the invention provides a preformed valve seat insert which is easily installed, easily removed and easily replaced to effectuate repairs and includes a hardened valve seat.